NCERT Curiosity · Class 9 · Science
Describing Motion Around Us
A truck ahead suddenly brakes — the safe distance to leave behind it depends entirely on speed.
Speed & Velocity Acceleration Graphs Kinematic Equations
Key idea
Distance and Displacement
Distance

The entire path length covered — a scalar, magnitude only.

Displacement

The net change in position — a vector, with magnitude and direction.

Key idea
A ball thrown up and caught at the same spot has zero displacement.
Its total distance is the trip up plus the trip down, added together — distance and displacement are only equal when an object never turns back.
Key idea
Average Speed and Average Velocity
Average speed

Total distance ÷ time — a scalar, no direction.

Average velocity

Displacement ÷ time — a vector, direction matches displacement.

Key idea
Uniform and Non-uniform Motion
Uniform

Equal distances covered in equal time intervals — constant speed.

Non-uniform

Unequal distances covered in equal time intervals — changing speed.

Key idea
Acceleration depends on how quickly velocity changes, not on speed itself.
An object moving very fast at a constant velocity has zero acceleration.
Key idea
Position-Time and Velocity-Time Graphs

Position-time

A straight line means constant velocity; a curve means changing velocity.

Velocity-time

A rising line means positive acceleration; a falling one means braking.

Key idea
Slope and Area Under the Graph
The slope

Slope = velocity on a position-time graph; slope = acceleration on a velocity-time graph.

The area

The area under a velocity-time graph equals the displacement.

Key idea
The Three Kinematic Equations

v = u + at

Final velocity related to time.

s = ut + ½at²

Displacement related to time.

v² = u² + 2as

Final velocity related to displacement.

Key idea
Circular Motion Is Still Accelerated
Motion in a plane

A kicked ball's path, or a vehicle overtaking another — confined to two dimensions.

Circular motion

Speed stays constant, but direction keeps changing — so the motion is still accelerated.

Chapter · key terms to remember
Key Terms

Distance / Displacement

Total path length / net change in position.

Scalar / Vector

Magnitude only / magnitude and direction.

Average speed / velocity

Distance / time / displacement / time.

Chapter · key terms to remember
More Key Terms

Uniform / Non-uniform motion

Constant speed / changing speed.

Average acceleration

Change in velocity ÷ time interval.

Slope

Velocity on a position-time graph; acceleration on a velocity-time graph.

Chapter · key terms to remember
More Key Terms

Kinematic equations

v=u+at, s=ut+½at², v²=u²+2as.

Uniform circular motion

Circular motion at constant speed.

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